Tar sands, also known as oil sands and bituminous sands, are sand deposits which are impregnated with dense, viscous petroleum. One method for recovering bitumen or heavy oil from such tar sands is the so-called hot water extraction process or hot caustic extraction process. In such a process, the tar sand feed is heated and mixed with water or water plus caustic to separate heavy oil or bitumen from sand. The oil/water/sand mixture is screened and introduced into settlers, one of which settles sand down to be removed as tailings, and the other of which floats the bitumen to be removed as froth. A mostly water middlings stream with some suspended fine mineral and bitumen particles is removed from the settlers, and mostly recycled to the extraction drum, except for a drag stream that is withdrawn as a purge to control the concentration of fines and contaminants in the middlings. After further treatment to recover additional bitumen and remove most of the sand, the aqueous tailings are sent to the final tailings pond.
The final tailings produced from the extraction of oil from a tar sands mining operation can contain an almost permanent dispersion of colloidal sand particles coated with bitumen in water. Since separating the solids and heavy oil from the wastewater can be nearly impossible, the aqueous tailings can be accumulated for future treatment in large retention ponds. Some of the tailings have been in tailings ponds for as long as forty years so far. Water from a typical final tailings pond can have a pH from 6 to 8, a dispersed solids content from 1 ppm by weight up to 25 weight percent or more, and a hydrocarbon content from 0.1 to 5000 ppm wt.
The tailings pond water that can be recycled to the extraction unit as process water has been limited because salts and other undesirable minerals accumulate therein. The tailings pond water can include hydrocarbons and other contaminants so that it cannot be introduced into waterways, or used in boilers to generate steam.
Similar wastewater streams including colloidal dispersions and/or oil emulsions can be produced from other mining operations, refineries and hydrocarbon upgrading operations. The waste waters from operations such as these described can be contaminated with clay, minerals and oils and can not be discharged to water sources such as rivers.
Steam can be employed downhole in wellbores for various purposes, such as to heat the petroleum and make the petroleum flowable either in the wellbore or from the formation. For example, steam can be injected into the bitumen containing tar sands in the bitumen or heavy oil production method known as steam assisted gravity drainage (SAGD). Steam can be injected in one or more injection wells completed in the heavy oil formation. The steam heats the heavy oil in situ, reducing the viscosity thereof and rendering the heavy oil flowable. The flowable heavy oil can then be produced at one or more production wells. However, obtaining water for production of injection steam can be difficult, and poses a potential problem for production of oil using SAGD production techniques.
In situ steam generators where fuel, oxidant and water can be supplied to a surface or downhole steam generator can more effectively produce steam at the desired location, thereby avoiding heat losses arising from distribution from a remote steam source to the wellbore, and in the case of subsurface steam generation, between the surface and the injection stratum.
A need exists for both for treatment and/or disposal of the tailings pond water from bitumen mining or similar sources, and for an alternative to a fresh water source to generate steam in situ for operations such as steam assisted gravity drainage (SAGD) processes and other steam assisted thermal processes for production of heavy oils.